1. Field of the Invention
The present invention relates, in general, to apparatus and methods for producing diamond crystals.
2. Description of the Art
High pressure/high temperature piston cylinder apparatus are used for a variety of purposes and, specifically, for the production of diamond crystals. In such apparatus, a core of charge material, such as graphite and metal catalyst solvent, is confined within a cylindrical reaction chamber and subjected to high pressure by advancing a moveable piston into one end of the cylindrical chamber, which is open. The other end of the cylinder is closed, and electrical insulation is placed between the cylinder and the backup support block. Electric current is then passed through the reaction charge to heat the sample. A more detailed description of the operation of such a piston-cylinder apparatus is given in U.S. Pat. No. 4,118,161 to Kennedy.
One of the problems associated with the manufacturing process for synthetic diamonds using the piston-cylinder device is uniform compression of a reaction charge with a high length-to-diameter aspect ratio. Sample geometries of this type are necessary for two reasons: first, long cylindrical reaction charges are very desirable because maintenance of a very uniform temperature throughout the sample is critical for production of high quality diamond crystals. As the sample length increases, the end effects of thermal losses are minimized. Second, increasing the sample length allows a higher volume of reaction charge to be processed in the apparatus, resulting in a cost effective method for synthetic diamond production.
It would be desirable to eliminate the closed end of the piston-cylinder apparatus and have two moveable pistons, so that by compressing the reaction charge from both ends, a more uniform sample deformation would occur resulting in isostatic pressure distribution and a uniform thermal profile throughout the sample. However, this cannot be accomplished by using piston-cylinder apparatus of the prior art. In prior art apparatus, one end of the piston-cylinder apparatus is closed in order to provide a means for electrically isolating the cylindrical core so that an electrical current can be passed through the reaction charge for the purposes of heating the sample. This insulation is necessary because the pistons and the cylinder are composed of very hard, ultra high strength steel or, preferably, cobalt cemented sintered tungsten carbide in order to withstand the very high pressures. The very close fit of the piston composed of an ultra high strength material does not permit the use of an insulating gasket, so the moveable piston is not electrically insulated from the bore. The snug fit of the pistons is required to keep the sample from squirting out during compression, thus resulting in an electrical short that interrupts the heating cycle when moveable pistons are used on both ends.
A prior art solution to this problem is to use the well known belt-type high pressure apparatus. Prior art apparatus of the belt-type have been described in numerous patents, such as U.S. Pat. No. 2,941,248 to Hall et al. In the belt-type high pressure apparatus, the pistons taper inwardly toward the charge and the ends of the cylinder, in which the charge is disposed, are correspondingly tapered. Gaskets formed of deformable electrical insulating material, such as pyrophyllite, are disposed between the tapered pistons and the tapered ends of the cylinder and seal the charge in the cylinder.
The gaskets must deform to permit the pistons to advance into the cylinder to compact the charge. A serious disadvantage is that as the tapered pistons move closer together, more and more of the force is supported by the cylinder (shoulder loading) between the side walls of the pistons and the upper and lower portions of the cylinder. This shoulder loading limits the amount of advance of the pistons and the force which can be maintained upon the reaction charge. This is especially problematic during the production of synthetic diamond because there is a significantdifference in the density of graphite and diamond; the density of diamond is about 3.54 while the density of graphite is only about 2.25.
Thus, when the sufficient temperature and pressure conditions are present and the graphite begins to convert to diamond, there is a corresponding reduction of volume in the core of charge material. It is desirable to maintain as constant a pressure as possible; therefore, the piston has to be advanced into the cylinder as the volume reduction is occurring.